Booleans and Comparison

A program makes decisions by asking yes-or-no questions: is the score above 100? Is the list empty? The answer to such a question is a Bool, which has exactly two values, true and false.

const is_raining = false
const game_over = true
print(is_raining, game_over)

Output

false true

Comparing values

A comparison asks a question about two values and gives a Bool:

Operator	Question	Example	Answer
==	equal?	3 == 3	true
!=	not equal?	3 != 3	false
<	less than?	3 < 5	true
<=	less than or equal?	5 <= 5	true
>	greater than?	3 > 5	false
>=	greater than or equal?	5 >= 3	true

const score = 120
print(score > 100)
print(score == 50)

Output

true
false

== has two equals signs: one = gives a variable a value, and two ask whether two values are the same.

Numbers compare as you’d expect, and an Int and a Float compare with each other, so 1 == 1.0 is true. Strings compare alphabetically with < and >, and exactly with ==, so "a" == "A" is false.

Asking whether a value is in a range

To ask whether a value is between two others, write the comparisons in a row:

const score = 75
print(0 <= score <= 100)

Output

true

That reads the way you’d say it, “score is between 0 and 100”, and means 0 <= score and score <= 100.

Combining answers

Three words combine Bools:

• a and b is true when both are true.
• a or b is true when at least one is true.
• not a is the opposite of a.

const age = 15
const has_ticket = true
print(age >= 13 and has_ticket)
print(age < 13 or not has_ticket)

Output

true
false

Emerald spells these as words. If you know a language that writes &&, ||, and !, those aren’t a second spelling here; != is still “not equal”.

and and or stop as soon as they know the answer. In false and anything, the answer is already false, so anything is never worked out:

func check(): Bool {
    print("checking...")
    return true
}

print(false and check())
print(true or check())

Output

false
true

check never runs, so checking... never prints. This matters when the right side would fail without the left: count > 0 and total / count > 10 never divides by zero, because a zero count stops it first. (Functions are explained on the functions page.)

Only a Bool is true or false

Some languages treat 0, an empty string, or an empty list as false. Emerald doesn’t: a condition has to be a real Bool, so the question is always written out.

const count = 3
if count {
    print("some")
}

Output

booleans.em:2:4: a condition must be a Bool, but this is Int
  if count {
     ^^^^^
Compare it to something, as in `count > 0`. Emerald has no truthy or falsey values.

Write if count > 0 instead. Likewise, values of different types can’t be compared, because the answer would only ever be false:

print(1 == "1")

Output

booleans.em:1:7: Int and String cannot be compared
  print(1 == "1")
        ^^^^^^^^
`==` and `!=` compare two values of the same type, and Int and Float compare with each other.

Convert one first: 1 == "1".to_int() is true.

Questions that end in ?

A method whose name ends in ? answers a yes-or-no question and gives a Bool:

print("".empty?())
print("Emerald".starts_with?("Em"))
print(7.even?())

Output

true
true
false

The ? is part of the name, and it is how you can tell at a glance that the answer is true or false.

What you’ve learned

• A Bool is true or false, the answer to a yes-or-no question.
• ==, !=, <, <=, >, and >= compare values; 0 <= x <= 100 asks for a range.
• and, or, and not combine answers, and and and or stop as soon as they know the answer.
• A condition must be a real Bool, and only values of the same type can be compared.
• A method ending in ? answers a yes-or-no question.

Next, operators: every operator in one place, and the order they are worked out in.